Apparatus for removing noble metal contamination from liquids

ABSTRACT

Silicon is employed as a reducing agent in an acid bath to adsorb noble metals present as contaminants in the acid. In the manufacture of silicon devices for electronic memory and other devices, polonium-210 is adsorbed by silicon getters to reduce soft error rate attributable to alpha particle emissions from the radioactive polonium. The noble metals in addition to polonium which can be plated onto silicon using the disclosed method are gold, silver, platinum, copper, palladium, mercury, selenium and bismuth.

This application is a continuation of U.S. patent application Ser. No.08/613,831 which was filed on Mar. 11, 1996, now U.S. Pat. No.5,691,211, which is a continuation of application Ser. No. 07/975,789,filed Nov. 13, 1992, now U.S. Pat. No. 5,501,767.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of removing noble metal contaminantsfrom a mineral acid bath, and more particularly, in the manufacturing ofpackaged semiconductor devices, a method for removing contaminant Po-210from a heated phosphoric acid bath using silicon as a getter.

2. Description of the Prior Art

In the process of manufacturing large scale integrated circuits (LSI)from a silicon substrate or wafer, phosphoric acid is often used as asolution to selectively remove silicon nitride.

Phosphoric acid commonly contains trace element of Polonium-210(Po-210). Po-210 is a radioactive element and a source of alpharadiation emissions. Levels of Po-210 and, in proportion, alphaemissions, vary depending on the content of Po-210 in phosphorous usedto make the acid solution. The Po-210 present in any deposit ofphosphorous is a function of the natural decay of U-238. Po-210 Levelsmay vary greatly from an imperceptible emission level to significantalpha emissions.

It is well know in the industry that alpha particle emissions are one ofseveral known causes of soft errors in LSI memory devices. Soft errorshave been defined to be random, non-recurring single bit errors inmemory devices. They are not permanent, i.e., no physical defects areassociated with the failed bit. A bit showing soft error is completelyrecovered by the following write cycle, for example, in a dynamic memorydevice where refresh of memory stored data occurs every severalnanoseconds.

Other identified causes of soft errors are system noise, voltagemarginality, sense amplifiers and pattern sensitivity, all statisticalpredictors of the rate at which soft errors will occur (SER).

Po-210, when present in phosphoric acid in the etching process, has beenidentified as a source of alpha particle emissions. Po-210 has anaffinity for Silicon (Si) in the acid bath and plates onto the surfaceof the Si wafers which are being etched for production of finished LSImemory and other devices. The Po-210 remains bonded at surface sites onthe silicon wafer, through later subsequent manufacturing steps. Thefinished LSI circuit or die results in a memory device with internallyemitted alpha particles. The alpha particles are emitted by the Po-210contaminating the die. Thus, the memory device becomes itself a sourceof contribution of SER.

SUMMARY OF THE INVENTION

Silicon is employed as a “getter” to attract Polonium 210 (Po-210)molecules and remove them from the liquid phosphoric acid bath. Getteris used in this instance to describe silicon as the attracting agentwhich reacts with the noble metal, Po-210 in this example. The mechanismfor this attraction is described as follows: The getter is the vehiclefor removing the contaminating noble metal. Gettering refers to the actof attracting and removing the trace elements through the reduction ofthe trace elements onto silicon. Noble metals include silver, gold,copper, platinum, palladium, mercury, selenium and bismuth, in additionto Polonium.

In accordance with the teachings of the present invention, a method ofremoving noble metal trace elements from a mineral acid is disclosed,comprising the steps of heating a liquid mineral acid in a container,inserting a silicon getter into the acid in fluid contact with the acid,and removing the silicon getter from said mineral acid.

A method is set forth, wherein the liquid acid is heated to atemperature of approximately 145-150 degrees C. The mineral acid isselected from the group consisting of phosphoric acid or sulfuric acid,and any acidic solution of pH 6 or lower. The mineral acid contains atleast a trace of a noble metal. The noble metal is selected from thegroup consisting of Polonium, gold, silver, platinum, copper, palladium,mercury, selenium and bismuth. The getter is inserted in the liquidmineral acid for at least thirty minutes.

A method of adsorbing Po-210 with a silicon getter from a phosphoricacid solution comprises the steps of providing liquid phosphoric acid ina container, heating the container of liquid phosphoric acid to between145 degrees C. and 150 degrees C., inserting at least one siliconarticle into an open vessel, placing the vessel into the acid bath atleast until reaching process temperature, soaking the silicon article inthe liquid phosphoric acid and removing the vessel from the container.

An improved method of manufacturing electronic semiconductor integratedcircuits is also disclosed. The method of substantially eliminatingα-particle emissions attributable to manufacturing materials, comprisesthe steps of providing a quantity of phosphoric acid having at least aportion composed of Po-210, then heating the quantity of phosphoric acidto a temperature suitable for removal of silicon nitride from a siliconwafer, inserting a silicon getter into the phosphoric acid during orfollowing the heating step, next removing the silicon getter from thephosphoric acid, and etching silicon nitride from a semiconductor waferhaving wiring paths defined thereon.

It is therefore an object of the present invention to provide a methodof eliminating alpha particle emissions from phosphoric acid whilesimultaneously removing Po-210 from the acid solution.

It is further an object of the present invention to provide a method ofdecontaminating phosphoric acid for use in the manufacture of siliconLSI memory devices.

Another object of the present invention is to provide a method ofplating noble metals onto silicon from an acidic solution.

Yet another object of the present invention is to provide a method ofmanufacturing a LSI memory device substantially free of soft errors dueto internal alpha particle radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a top view of the process tank placed within areservoir tank adjacent to pump and filter.

FIG. 2 illustrates the top view of FIG. 1 with a cover placed over topof the pump and filter sections.

FIG. 3 is a cross-sectional profile view taken along the lines 3—3 inFIG. 1.

FIG. 4 is an illustration of a vessel containing silicon wafers.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following experiments were devised to illustrate and to prove theeffectiveness of the present invention.

EXPERIMENT NO. 1

In this experiment, a set of four circular silicon wafers measuring 6″in diameter and 675 microns in thickness were dipped in a bath of freshphosphoric acid at 145 degrees to 150 degrees C. for thirty minutes. Thefirst four wafers were then removed. [The term “wafer” as used in thisdescription of this invention refers to a wafer of the type used in themanufacturer of packaged semiconductor device. The dimensions are 6″diameter circular silicon wafer of 650 to 700 micron thickness.]

The phosphoric acid bath was then gettered by dipping a batch of fiftysilicon wafers for thirty minutes. The temperature was held constant at145°-150° C. The fifty wafers used to buffer the acid bath are definedas getters. The getters were then removed.

A second group of four wafers was then dipped in the same bath for 30minutes at the same temperature.

α-particle emissions were next measured by placing the first group offour wafers in a cloud chamber for nine hours. A reading of thecumulative α-particles counts was taken once every hour for nineconsecutive hours. Measurements were made using a Spectrum Science Co.Model 850 radiation analyzer. The cloud chamber was purged. Then thesecond group of four wafers was placed in the cloud chamber. α-particleemissions were measured using the same procedure. The results are setforth in Table 1.

TABLE 1 α-particle After α-Particle Emissions Group 2 after Hour Group 1before buffering buffering 1 382 209 2 707 400 3 1053 585 4 1371 774 51681 971 6 2007 1161 7 2312 1353 8 2617 1518 9 2919 1706 Average Counts77.4 46.6 per wafer-hour

EXPERIMENT NO. 2

Eight gallons of phosphoric acid was mixed in solution. Half was placedin one tank and the remainder in a second tank. The mixture was dividethis way in order to ensure that the chemical reagents in each tank wereidentical. Both tanks were then gradually heated to 145° to 150° C.,over approximately a thirty minute period.

The first tank was not treated with getter wafers before introducing thesample production wafers. Two sample production wafers were placed inthe heated phosphoric acid bath for thirty minutes and then removed.α-emission measurements were made after placing the wafers in a cloudchamber. Using a Spectrum Science Model 850 radiation detector,measurements were taken at one hour intervals for nine hours. Theresults are listed in Table 1A.

The second tank was first gettered with fifty getter wafers. Afterheating to the desired process temperature above, the getter wafers wereplaced in a vessel and submerged in the phosphoric acid bath of tank #2.After thirty minutes elapsed, the vessel was removed. Then two sampleproduction wafers were placed into the buffered phosphoric acid bath.Alpha emissions were then measured in the same manner as described forthe first tank sample wafers. The results are listed in Table 1B.

The average counts per wafer-hour was calculated as a measure ofradioactivity level for comparison purposes. The calculation is made bysubtracting the 4th hour cumulative emissions from the 9th hourcumulative total, in order to eliminate any possible “background” orambient radioactive contribution due to other sources. Radon gas is oneexample of ambient background radiation contribution. The difference inthose two readings yields a 5-hour cumulative total α-emission which issolely attributable to contaminants in the process material—i.e.—Po-210present in the phosphoric acid. The total is then divided by 5 to yieldan average hourly emission, then divided again by two to yield aper-wafer, per hour measurement.

TABLE 1A - TABLE 1B Alpha Particle Alpha Particle Emissions MeasureAfter Emissions Measured (Acid Bath Hour No. (Acid Bath Not Buffered)Buffered) 1 149 counts  68 counts 2 234 153 3 346 224 4 460 304 5 565368 6 665 427 7 772 501 8 861 563 9 950 635 Average 49 counts/wafer-hour33.1 counts/wafer hour

This experiment reveals that emissions from sample production wafers intank two after buffering measured approximately 65% of the emissionlevels of the identical wafers from tank one.

Each tank contained identical solutions of phosphoric acid, and thesample production wafers tested in each tank were selected from the sameproduction batch.

A second part of the same experiment consisted of measuring the alpharadiation levels of the getter wafers—the group of 50 wafers used tobuffer tank number two. Two wafers (No. 9 and 10) were randomly selectedfrom this group and measured for alpha radiation using the samemeasurement procedure as used in the first part of this experiment.Table 2A sets forth the results of this phase of the experiment.

TABLE 2A After Alpha Hour Emission No. Counts 1 105 2 186 3 269 4 362 5438 6 531 7 602 8 697 9 784

Consistent with the results in tables 1A and 1B, this establishes thatthe source of alpha emissions is gettered by the Si wafers placed in thevessel. While the Po-210 is adsorbed by fifty of the gettering wafers ina half hour soak period, it is expected that the larger number pervolume of acid of getter wafers will adsorb Po-210 a rate (42.2) equalto or less than the two wafers placed in the unbuffered solution of tankNo. 1(49).

EXPERIMENT NUMBER 3

In a third experiment, the effect of gettering as a function of time wasdetermined. Results showed that the longer the silicon gettering waferswere immersed in the heated acid bath, the lower the Po-210contamination in the acid solution after the gettering wafers areremoved.

A tank of liquid phosphoric acid was heated to 145°-150° C. From a batchof fifty wafers, two were selected at random (Nos. 13 and 14). These twowafers were submerged for thirty minutes and removed, then cleaned,dried and stored in one airtight flat-pack container for a short time.Using the same measurement technique, α-particles were measured at 45.5counts per wafer-hour.

The same acid bath at process temperature was then gettered by insertingthe vessel into the bath for one hour. The vessel contained fiftygettering wafers. After one hour, the vessel was pulled from the tankand two production wafers were placed in the tank for one half hour.α-emissions for these two wafers were measured at 32.4 counts perwafer-hour.

The same acid bath was again gettered with a second batch of fiftysilicon wafers for a second one hour interval. The vessel containing thegetters was removed. Two production wafers were inserted in the bath forone half hour and removed. α-particle emissions were recorded as 26.6counts per wafer hour.

By buffering the acid bath with fifty getter wafers for one hour a 28.8%reduction in α-particles was realized; after then buffering the samebath for a second hour, an overall reduction in α-particle emission of41.5% was measured.

EXPERIMENT 4

In order to determine the relation of surface area of the silicon gettermaterial to the efficiency of α-particle (Po-210) removal, a fourthexperiment was conducted. In this experiment, a quantity of high-qualitysilicon beads weighing 500 grams were used instead of the 50 wafers(weight=1500 grams) as in the previous experiments. The silicon beadsare spherical, so it was assumed—without measuring—that these wouldpermit significantly greater fluid contact with the phosphoric acid.

A batch of phosphoric acid was mixed and divided into two separatetanks, then both tanks were heated to 145°-150° C. One tank was bufferedwith silicon getter beads for approximately four to five hours atprocess temperature. The second tank was not buffered at all, andremained at process temperature for the same time interval.

Five production wafers were then placed in each tank and removed afterthirty minutes. The wafers were then placed in the cloud chamber.α-particles were measured again with the Spectrum Model 850 radiationanalyzer. For this experiment, measurements were recorded once everyhour for fifteen consecutive hours. Contribution due to backgroundemissions were compensated by subtracting hour 4 reading from hour 15reading. The counts per wafer hour were determined by dividing thedifference by eleven (hours); and dividing again by five (wafers)[according to the same method as experiment number 1] to arrive at thecomparison figures. Table 4 shows the readings for each set of wafersaccording to each of fifteen hours.

TABLE 4 Hour Unbuffered Buffered  1 1008 168  2 2004 290  3 2953 408  43879 514  5 4845 607  6 5793 722  7 6755 607  8 7682 722  9 8630 828 109553 1171 11 10,507 1282 12 11,459 1412 13 12,399 1542 14 13,352 1658 1514,286 1768 Counts per 189.2 22.8 wafer-hour

Results indicated reduction of α-particle emissions of over 80% in thesilicon wafers after buffering the phosphoric acid bath compared toα-particle emissions in the silicon wafers when the acid bath was notbuffered.

It should be noted that each batch of phosphoric acid may have high orlow Polonium-210 content, depending on the source where the phosphorouswas mixed. Thus, the magnitude of the various readings, for buffered andunbuffered results is less significant than their percentagedifferences.

Referring to FIGS. 1, 2 and 3, a preferred embodiment of the presentinvention is shown. Recirculating bath filter system 10 includesreservoir tank 12. Heater 14 is located adjacent to the bottom of tank12. Heater 14 may be of insertion type or immersion type. Process tank16 fits into reservoir 12. Reservoir 12 and process tank 16 arepreferably made of teflon to withstand the corrosive liquids containedwithin them.

Reservoir tank 12 is filled with liquid phosphoric acid 20, then heatedto 145-150° C. The process temperature ideally suited to strippingnitride from the surface of silicon when etching microscopic circuitsonto silicon wafer surfaces. Filter housing 24 is located in filter tank30. During the heating cycle and after, phosphoric acid 20 is circulatedthrough silicon media 28 in filter housing 24. Pump 22 circulates acid20 through filter housing 24 and back to reservoir 12 through conduits26, 26A (not shown) made of teflon fittings for supply and return ofacid 20.

Silicon media 28 getters po-210 from the phosphoric acid. After apredetermined interval has elapsed—usually thirty to forty-fiveminutes—process temperature in the bath is achieved and Po-210 isgettered from the acid. Vessel 32 is substantially immersed in theliquid acid bath 20 by placing it into process tank 16. Vessel 32contains silicon wafers 34 which are then cleaned and etched formanufacture of integrated circuit devices. Note the distinction betweengettering wafers 36 and silicon wafers used in production of finaldevices.

Referring next to FIG. 4, vessel 32 is illustrated for use in analternative method of gettering Po-210 from phosphoric acid. In thismethod, gettering wafers 36 are placed in vessel 32 then submerged inreservoir tank 12 while the bath is heated to process (145°-150° C.)temperature from ambient. This method is useful where it may beimpractical or uneconomical to equip tank 12 with filter housing 24 andpump 22, and associated plumbing. Gettering wafers 36 replace siliconmedia 28 from filter housing 24, and are manually inserted and removedfrom the acid reservoir 20 to getter Po-210 before the productionsilicon wafers are dipped in reservoir 20. Vessels 32 are shaped like adish rack so that the flat gettering wafers 36 may be supportedvertically in a row, typically twenty five to a vessel.

As previously described, noble metals in an acidic solution will beadsorbed by—or plate onto—a reducing agent such as silicon. Forsubstantial plating to occur, the pH of the solution must be 6 or lower.Optimal plating conditions exist in a mineral acid bath such asphosphoric or sulfuric. While heating the bath increases the rate atwhich noble metals are adsorbed by the reducing agent, it is notnecessary to heat the acid solution in order for adsorption to occur.

Similarly, the time of exposure of the silicon reducing agent is setforth in the experiments above, as a means of illustrating effects andfor measuring the effects. It should be noted that reduction of thenoble metal begins immediately at room or ambient temperature, so longas the acid is liquid to allow for immersion of the reducing agent.

Depending on the quantity of acid solution one desires to decontaminateof noble metal traces, temperature and time of exposure may be increasedor decreased to yield timely and efficient results. Experiments onethrough four above were conducted with quantities of eight gallons ofphosphoric acid. The same effect will be realized on much greaterscales. For example, in a large storage facility for storing acid, itmay be impractical or uneconomical to heat large storage tanks. It isnot necessary however to add heat where the silicon may be exposed forsignificantly longer intervals.

According to the provisions of the patent Statutes, we have explainedthe principal, preferred construction and mode of operation of ourinvention and have illustrated and described what we now consider torepresent its best embodiments. However, it should be understood that,within the scope of the appended claims, the invention may be practicedotherwise than as specifically illustrated and described.

We claim:
 1. A gettering device for removal of metallic elementsselected from the group consisting of polonium, gold, silver, platinum,copper, bismuth, palladium, and mercury from a liquid, the devicecomprising: a reservoir tank for holding the liquid; at least one vesselremovably inserted in the reservoir tank; a plurality of getteringwafers removably inserted in the vessel for submergence in the reservoirtank; and a heater coupled to the reservoir tank.
 2. A gettering devicefor removal of at least one metallic element from a liquid, the devicecomprising: a reservoir tank for holding the liquid; at least one vesselremovably inserted in the reservoir tank; a plurality of silicon wafersin a filter housing removably inserted in the vessel for submergence inthe reservoir tank; and a heater coupled to the reservoir tank.
 3. Agettering device for removal of polonium-210 from a liquid, the devicecomprising: a reservoir tank for holding the liquid; a filter housingcontaining a silicon medium; a pump connected to the reservoir tank andthe filter housing for circulating the liquid from the reservoir tank tothe filter housing and back to the reservoir tank; and a heater coupledto the reservoir tank for heating the liquid to a temperature suitablefor attracting the polonium-210 to the silicon medium.
 4. The getteringdevice of claim 3 wherein the silicon medium of the gettering devicefurther removes metal trace elements selected from the group of metalsconsisting of polonium, gold, silver, platinum, copper, bismuth,palladium, and mercury.
 5. The gettering device of claim 3 wherein theliquid has a pH of approximately 6 or lower.
 6. The gettering device ofclaim 3 wherein the heater is adapted to heat the liquid to atemperature of about 150 degrees centigrade.
 7. The gettering device ofclaim 4 wherein the heater is adapted to heat the liquid to atemperature within a range of about 145-150 degrees centigrade.
 8. Thegettering device of claim 3 wherein the liquid is selected from thegroup consisting of phosphoric acid and sulfuric acid.
 9. The getteringdevice of claim 3 wherein the reservoir tank is made of teflon.
 10. Agettering device for removal of metal trace elements selected from thegroup of metals consisting of polonium, gold, silver, platinum, copper,bismuth, palladium, and mercury from an acidic liquid, the devicecomprising: a reservoir tank for holding the acidic liquid; a filterhousing containing silicon beads; a pump connected to the reservoir tankand the filter housing for circulating the acidic liquid from thereservoir tank to the filter housing and back to the reservoir tank; anda heater coupled to the reservoir tank for heating the acidic liquid toa temperature within the range of 145-150 degrees centigrade.
 11. Thegettering device of claims 10 wherein the acidic liquid has a pH ofapproximately 6 or lower.
 12. The gettering device of claim 10 whereinthe acidic liquid is selected from the group consisting of phosphoricacid and sulfuric acid.
 13. The gettering device of claim 10 wherein thereservoir tank is made of teflon.
 14. A gettering device for removal ofmetal trace elements selected from the group of metals consisting ofpolonium, gold, silver, platinum, copper, bismuth, palladium, andmercury from an acidic liquid, the device comprising: a reservoir tankfor holding the acidic liquid; at least one vessel removably inserted inthe reservoir tank; a plurality of gettering wafers removably insertedin the at least one vessel such that when the at least one vessel isplaced in the reservoir tank, the plurality of gettering wafers aresubmerged in the acidic liquid; a heater coupled to the reservoir tankfor heating the acidic liquid to a temperature within the range of145-150 degrees centigrade.
 15. The gettering device of claim 14 whereinthe plurality gettering wafers are flat and circular and the at leastone vessel is constructed and arranged to support the plurality ofgettering wafers vertically in a row.
 16. The gettering device of claim14 wherein the acidic liquid has a pH of approximately 6 or lower. 17.The gettering device of claim 14 wherein the acidic liquid is selectedfrom the group consisting of phosphoric acid and sulfuric acid.
 18. Thegettering device of claim 14 wherein the reservoir tank is made ofteflon.
 19. A gettering device for removal of metallic elements from anacidic liquid, the device comprising: a reservoir tank for holding theacidic liquid; at least one vessel removably inserted in the reservoirtank; a plurality of gettering wafers removably inserted in the vesselfor submergence in the reservoir tank; and a heater coupled to thereservoir tank.
 20. The gettering device as recited in claim 19 whereinthe gettering wafers are made of a silicon medium.
 21. The getteringdevice as recited in claim 20 wherein the silicon medium further removesmetal trace elements selected from the group consisting of polonium,gold, silver, platinum, copper, bismuth, palladium and mercury.
 22. Thegettering device as recited in claim 20 wherein the heater is capable ofheating the acidic liquid to a temperature of about 150 degreescentigrade.
 23. The gettering device as recited in claim 20 wherein thereservoir tank comprises teflon.
 24. A gettering device for removal ofpolonium-210 from a liquid, the device comprising: a reservoir tank forholding the acidic liquid; at least one vessel removably inserted in thereservoir tank; a plurality of gettering wafers removably inserted inthe vessel for submergence in the reservoir tank; and a heater coupledto the reservoir tank.
 25. The gettering device as recited in claim 24wherein the heater is capable of heating the acidic liquid to atemperature suitable for attracting polonium-210 to the silicon medium.26. A gettering device for removal of metal trace elements selected fromthe group of metals consisting of polonium, gold, silver, platinum,bismuth, palladium, and mercury from an acidic liquid, the devicecomprising: a reservoir tank for holding the acidic liquid; a filterhousing containing silicon beads; a pump connected to the reservoir tankand the filter housing for circulating the acidic liquid from thereservoir tank to the filter housing and back to the reservoir tank; anda heater coupled to the reservoir tank for heating the acidic liquid toa temperature within the range of about 145-150 degrees centigrade. 27.The gettering device of claims 26 wherein the acidic liquid has a pH ofapproximately 6 or lower.
 28. The gettering device of claim 26 whereinthe acidic liquid is selected from the group consisting of phosphoricacid and sulfuric acid.
 29. The gettering device of claim 26 wherein thereservoir tank is made of teflon.